The present invention relates to a diode laser with spectrally selective feedback by means of a volume Bragg grating; in particular, the present invention relates to a low-noise, wavelength-stabilized diode laser with collimated radiation and a small beam diameter.
The radiation of edge-emitting diode lasers is highly divergent in a direction perpendicular to the waveguide plane (vertical direction, “fast axis”) and has a comparatively broad wavelength spectrum. In addition, the wavelength spectrum typically depends on further parameters, such as the temperature. As a result, the wavelength spectrum depends on the power supplied by the laser
To collimate the radiation in the vertical direction, cylindrical lenses with short focal lengths, so-called fast-axis collimators (FAC), are mostly used. In this case, the beam diameter of the collimated beam is proportional to the divergence of the radiation emitting from the diode laser and to the focal length of the vertically collimating lens. According to the state of the art, the wavelengths can be limited and stabilized by means of internal or external wavelength-selective elements or structures.
An external limitation and stabilization of the wavelengths is achieved as a result of spectrally selective feedback of the emitted radiation into the diode laser. An example are so-called external-cavity diode lasers (ECL) where feedback is done by means of spectrally selective reflection on surface gratings. This, however, has the drawback that additional optical elements are required and miniaturization is made difficult.
Another way to achieve spectrally selective feedback is the use of volume Bragg gratings. The advantage of using such volume Bragg gratings is that compact, wavelength-stabilized laser beam sources can be implemented.
For example, US 2005/0207466 A1, US 2006/0251143 A1, U.S. Pat. No. 7,298,771 B2, U.S. Pat. No. 7,397,837 B2 and U.S. Pat. No. 7,545,844 B2 teach how to place a planar volume Bragg grating in the vertically collimated or vertically and laterally collimated beam.
As an alternative, a planar volume Bragg grating can be placed in the divergent beam in front of the fast-axis collimator and vertical and lateral collimation is not done until the radiation has been transmitted through the volume Bragg grating, as is also known from US 2005/0207466 A1, US 2006/0251143 A1, U.S. Pat. No. 7,298,771 B2, U.S. Pat. No. 7,397,837 B2 and U.S. Pat. No. 7,545,844 B2.
In addition, it is known to manufacture the vertically collimating cylindrical lens (i.e. the fast-axis collimator) itself integrally with the volume Bragg grating, thus producing a single optical element which does both vertical collimation of the transmitted radiation and spectrally selective feedback.
If the volume Bragg grating is positioned behind the vertically collimating lens, there is the drawback that the desired small distance of the grating from the emitting surface (hereinafter also referred to as exit facet of the diode laser) is not possible, due to the necessary axial extension of the vertically collimating lens. It has been found, however, that placement of the volume Bragg grating close to the emitting surface is an essential requirement for low-noise operation of the diode laser.
If the volume Bragg grating is positioned between the emitting surface and the vertically collimating lens, the distance between the emitting surface and the grating can be made small, thus ensuring low-noise operation. In this case, however, the vertical beam diameter of the collimated beam has a lower limit, due to the axial dimensions of the volume Bragg grating and the vertically collimating lens. There are minimum required axial dimensions of the grating due to functional reasons and of the lens due to manufacturing reasons, and this entails the drawback of a non-negligible minimum size of the vertical diameter of the collimated beam.
If the volume Bragg grating itself is designed as a single vertically collimating lens, the desired small distance of the grating from the emitting surface must correspond to the back focal length of the grating/lens unit. For proper collimation, the grating/lens unit must be positioned within the back focal length from the emitting surface. Due to manufacturing tolerances, however, the back focal length varies to a degree that can be much greater than the desired distance of the grating from the emitting surface. In particular, the back focal length can become negative, so that collimation is no longer possible.
In summary, none of the methods and devices mentioned above enables the volume Bragg grating to be placed as close as possible to the emitting surface to ensure low-noise operation on the one hand while also placing the vertically collimating lens as close as possible to the emitting surface to ensure a sufficiently small vertical diameter on the other. In particular, one of the components cannot be placed close enough to the emitting surface if two components are used (planar volume Bragg grating and cylindrical lens). If a single optical element incorporating the cylindrical lens and the volume Bragg grating is used, the existing manufacturing tolerances, combined with the requirement of a very small vertical beam diameter, may result in a negative back focal length, so that collimation is no longer possible.
Therefore, the object of the invention is to provide a diode laser with wavelength stabilization and vertical collimation of the emitted radiation, which allows a small distance of the volume Bragg grating from the emitting surface, a small vertical diameter of the collimated beam and also compensation for manufacturing tolerances affecting the shape of the grating and the lens.